1. Field of the Invention
This invention concerns material exhibiting large non-linear optical properties, and relates in particular to such materials that are in the form of flexible rod-like polymeric compounds.
2. Description of the Technology and its Application
Most common materials transparent to light, such as ordinary glass, have the property of bending, or deviating, the path of light as it passes into and out of the material across the interface between the material and the surrounding medium, a process known as refraction, and a measurement of the magnitude of this deviation is given by the material's refractive index. Most such materials have a refractive index which is a constant with the light level, but some exhibit rather strange behaviour when refracting light, in that the degree of deviation varies depending upon the intensity of the light. These latter materials are said to have a non-linear optical (NLO) response, also known as the .chi..sup.(2) response (the material being a .chi..sup.(2) material). In effect, for an NLO medium the refractive index increases with the light amplitude, so that a prism of such a material would refract an intense beam of light more than a weak one. The precise definition of .chi..sup.(2) is in the connection between induced polarisation P and the square of the field strength E, that is: EQU P=.chi..sup.(2) E.sup.2
(for simplicity, and because it adds little to the discussion in the present context of uniaxial media, the tensor character of this relationship is here ignored). Light is an electromagnetic wave, having an electric field component, and the reason that refractive effects, whether linear or non-linear, happen at all is connected with the manner in which the light's electric field interacts with the electrons in the chemical bonds of the very molecules of which the material is composed.
A number of NLO materials are well known, and have been prepared for use in various types of optical apparatus and equipment that have been designed to take advantage of their unusual behaviour. These optical devices exploiting NLO effects fall into three main categories, thus:
(1) Modulators, which superimpose a signal onto an optical carrier wave for communications purposes. The very high frequency of light means that an extremely large bandwidth is achievable, and this implies immense communications potential. PA1 (2) Switches, which combine light beams, and in so doing change the beam's direction. These can be used for "chopping" a beam, or for combining beams in the logic gates of an optical computer (one that carries its signals in the form of light pulses rather than electronically). PA1 (3) Frequency doublers, which provide an output signal of double the frequency of the input signal, and employ a combination of tuned and non-linear circuitry. These can be used, for example, to shift the output of a "red" laser to the blue end of the visible spectrum. PA1 (1) the dipoles are larger (thus making the E field more effective); PA1 (2) the molecular environment restricts angular freedom even when the material is in a "fluid" state, for then the poling field does not have to work against an unfettered tendency to disorder (and this restriction of freedom can be arranged by dispersing the dye molecules within a continuous phase that is nematia in character); and PA1 (3) the glassy (solid) phase is highly stable against subsequent re-organisation of the molecules, and thus against any loss of the dipolar order needed for the NLO response. PA1 (1) One way of increasing the size of the effective dipole has been to polymerise a number of dipolar monomers in such a way that in the polymer all the individual dipoles are regularly aligned head-to-tail--that is, pointing in the same direction. A long polymeric rod-like molecule such as poly(.gamma.-benzyl-L-glutamate), commonly known as PBLG, can have a huge overall dipole (since the dipole on each monomer happens to point along this particular molecular rod in the same sense). The measured NLO response of PBLG confirms this idea; see B. F. Levine and C. G. Bethod, J. Chem. Phys. 65, 1989 (1976), who show that the monomeric dipoles, in acting together in each polymer molecule, together couple strongly to the poling field; the up/down imbalance thus created is also that which allows the NLO elements, the ".beta.s", to act coherently with each other. The NLO response is high in the sense that it is proportional to the degree of polymerisation (number of monomers per polymer) which can be a large number, 10-500 for instance. It is by this factor that the .chi..sup.(2) response is higher than that of conventional, monomeric materials. However, a serious problem with the use of such long, rigid rods, is that only in dilute solution do they have the freedom to move in response to the applied electric poling field (E.sub.poling) ; in a concentrated solution, or in a melt, the molecules are so close one to another that they physically interfere with each other, causing the "log jams" that prevent their reorientation. Additionally, PBLG forms a nematic phase; nematic forces (known as the "nematic mean field" or simply the "nematic field" and deriving from the nematic potential felt by molecules in a nematic phase of a fluid) act on the molecules, and make the intermediate positions (across the field) between the down and up estate highly unfavourable, so slowing the transition from down to up (that is, the poling process itself). For these two reasons complete poling of a concentrated PBLG would take an impractically long time, possibly of the order of several years. PA1 (2) Although there are disadvantages (as just discussed) where there is employed a nematic phase NLO material, there can also be advantages. At the molecular level a nomadic fluid consists typically of rod-like molecules. In contrast to a conventional fluid, where the molecules are largely randomly arranged, nematics have their rods orientationally ordered, pointing on average up or down (in equal proportions). This ordered aspect, which is the orientational equivalent to the ordering of atoms or molecules in a conventional crystalline solid, gives the "crystal" part of the characterising name "Liquid crystal", but since the rods are positionally as disordered as the atoms or molecules in a true fluid a nematic will still flow--hence the "liquid" part. Polymer liquid crystals are materials where the basic monomer constituents are nematic-forming molecolar rods. They are linked together to form a much larger molecule--a polymer--though still with the propensity to orientational order. PA1 (3) Polymers form easily the best glasses--that is, glasses with good mechanical and dielectric properties such as stability of molecular order, thin film and coating formation, and lack of brittleness.
For the most part, devices such as these, relying for their operation on the NLO response of a unit made from a light-refracting transparent medium, are only truly feasible, and powerful, if firstly their switching of a beam--that is, establishing the NLO effect after exposure to light--is extremely fast, and secondly there is only a very low absorbsance of the light in traversing the NLO medium unit (in effect the switch or device generally); the thinner the layer of the NLO material in a device that the beam must traverse the less loss occurs, but equally the thinner the layer the more powerful must be the NLO response.
3. Description of the Prior Art and its Problems
The various NLO materials proposed so far have for the most part been inorganic, and these do not meet the criteria of speed and low loss. They are both slow to react and lossy; this is because their response to the electrical field of a light beam involves the relative movement of the ions from which they are made, and not only do ions have considerable inertia but they are closely coupled to their neighbors (hence the sluggishness and the losses). It has been suggested, however, that there could be utilised organic NLO media which rely for their effect on their molecules containing electron-donating and electron-accepting regions that readily exchange electrical charge when acted upon by an electrical field. Here it is the electronic transitions within the molecule that determine the NLO response, and not only is this mechanism lossless but in addition it is very much faster (about 10.sup.4 times) than that involved when using inorganics. The intrinsic response at the molecular level is known as "the .beta. value", and molecules with such a response are commonly colored (the selective absorption of some frequencies of light to give a material with a net color is connected with the ability of its bonding electrons to transfer along the molecule), and so will often be useful as dyes.
There are two main problems involved in the selection and/or design of dyes suitable for use in NLO effect materials. Firstly, there is a need to optimise the .beta. value of the dye molecule by selecting both the best electron Donor and Acceptor groups and their relative positions and connections within the molecule. Secondly, there is the need, for molecules with a given .beta. value, to optimise the organisation of all the individual dye molecules in the solid phase to maximize the NLO response of the dye medium as a whole. In connection with this latter point, the molecules must be so ordered--aligned--that there is a net number pointing in one direction. Put another way, there should be an imbalance between the number pointing "up" compared with the number pointing "down", and this should be as large as possible; the medium then lacks "centro-symmetry", or it can be said that "dipolar order" has been achieved. When this is achieved, a majority of Donor-Acceptor (hereinafter "D-A") pairs on different molecules will point in the game direction, and so will act coherently together when the electric field of a light wave is applied, and thus there is a large net NLO effect. When there is no such up/down imbalance, the overall effect is zero. Naturally, once the mobility of the dye molecules in the fluid has enabled an external agent to create the poling up/down imbalance, their subsequent immobility in the solid state should ensure that, once so ordered, they will remain so, and thus the NLO effect will persist.
For the organic NLO materials used or suggested for use, various ploys and techniques have been applied to deal with these problems, and much work has been carried out, especially in relation to creating the net balance. In principle, one way to create the desired imbalance between up and down molecules is by poling--that is, applying a static electric field to the molecules when they are in a fluid (generally liquid) form. The permanent molecular dipoles (often in fact associated with D-A pairs that cause the NLO response), and hence the molecules themselves, are then forced by the applied field into partial alignment therewith, creating the necessary up/down imbalance required for an NLO response. Then, fixing the molecules in this aligned order by cooling the whole until it becomes a solid (usually a glass), when molecular motion on a large scale ceases, makes the imbalance, and thus the NLO response persist even when the poling field is removed.
Poling is more effective if:
In the work done in this field these three steps have been investigated and applied many times, both separately and in various combinations. The present invention also combines them, but in a novel and highly effective way that significantly amplifies their mutual effect.
A summary of the previous work carried out with the above three possibilities (large dipoles, movement-restricting environment, and stable solid phase) is roughly as follows:
On the other hand, when dilute solutions are employed to render the poling times acceptably short, the optical density of the material--the number of NLO elements per volume--is far too low to be useful, and moreover such dilute solutions are not at all easy to form subsequently into stable, solid glasses in which the poling-attained order is fixed into place. In short, the use of rigid rod polymers, like PBLG, has no device potential.
Where there is employed an NLO material having a nematic phase, that phase results in the restriction of the angular freedom of the molecules to being more nearly up or down rather than simply randomly orientated. Thus, when the poling E field is switched on, its effect in creating imbalance is theoretically up to five times larger for a hemarid phase material than for a non-nematic phase material (in the latter the orientational disorder is much harder for the field to overcome). The maximum figure obtains when the nematic order is perfect, and the molecules can only be up or down.
Using a nematic phase means that "field annealing" is necessary--that is, the nematic phase must be subjected to AC or DC fields to remove the textures and defects in the nematic phase (the discontinuities mentioned above) that would otherwise cause loss of light by scattering.
The use of the first of these three features--a polymer containing numerous aligned dipoles--has been widely investigated, but because so far the materials have in practical terms been unpolable this approach has generally been abandoned. Most consideration is accordingly given to the other two features, where it has been felt in particular that combinations should be advantageous. For example, if a good glass-forming polymer can be made nematic rather than isotropic then poling is more effective, and if the NLO dye itself can be part of the polymer rather than being dissolved/dispersed in the polymer matrix then more can be put in, thus increasing the optical density (and the dye can then be made as immobile as the polymer itself in the glassy phase, another important advantage).
Much effort has been directed at designing polymer liquid crystals, PLCs, for use in the NLO field, which meet these two criteria (nematic and dye-inclusive). One approach has been to incorporate the dye molecules in a comb, or side-chain, polymer liquid crystal--that is, a polymer wherein the nematic portions (rods) are attached to the polymer backbone rather like the teeth of a comb are attached to the comb's spine. The teeth, whether they be true NLO elements or simply nematic-forming portions, are attached via semi-flexible (generally aliphatic) "spacers" or "hinges". Such PLCs are described by, for instance, Celanese (DeMartino et al) in their EPO Publications Nos. 230,898, 231,770 and 235,506, by Thomson-CSF (Le Barny et al) in their EPO Publication No. 244,288, and (assigned to Hoechst Celanese) by DeMartino in U.S. Pat. No. 4,779,961.
However, these strategies have been only moderately successful. Thus, the enhancement due to nematicity is at most a factor of 5 (and in practice it is very much less than this), and is nothing like the huge effect motivating the PBLG (rigid rod) method because the rods inside the chain polymer molecules are weakly coupled to each other. Moreover, as the teeth of a side-chain polymer, the dye molecules still have some limited freedom in the glassy state, where it is primarily the motion of the polymer backbone (the main chain) that has ceased. Thus, such side-chain polymers result in a decay of up/down imbalance that should be fixed in.